1. Field of the Invention
This invention relates to a lockup clutch of a fluid type torque converter apparatus.
2. Description of the Prior Art
A fluid type torque converter apparatus is known as a device incorporated in a drive force transmission system of an automobile. Such a fluid type torque converter apparatus generally includes; an input shaft, an output shaft, an pump impeller, a turbine runner and stator, the pump impeller and turbine runner and stator being incorporated between the input shaft and the output shaft, and a lockup clutch built in between the input shaft and the output shaft and selectively connecting the input shaft and the output shaft to each other. The lockup clutch of the fluid type torque converter apparatus of the type includes a circular clutch disc fitted into a piston, and circular driven discs which are disposed on the opposite sides of the circular clutch disc, and one of which is connected to a turbine shaft which is the output shaft. The clutch disc and driven discs are assembled by rivets relatively rotatably, with the aid of two or more compression coil springs extending circumferentially of and over the clutch disc and these two driven discs. The relative rotation of the driven discs to the clutch disc is permitted only within the range of the length of circumferential slots provided in the clutch disc. The rivets respectively consist of a diametrically large portion piercing through each of circumferential slots provided in the clutch disc on the same radius thereof and held between the two driven discs when assembled, and diametrically small end portions fitted into the rivet holes provided in the driven discs. These rivets fasten one driven disc, the clutch disc and another driven disc to one another in the order described. The diametrically large portion of each rivet engages one longitudinal edge of respective slot provided in the clutch disc, for the centering of the clutch disc, and is adapted to slidingly move along the one longitudinal edge of each slot when an inputted torque changes or the piston engages the housing. For this reason, the diametrically large portion of individual rivet must be subjected to a heat treatment for hardening of the outer peripheral surface thereof. To apply the heat treatment to the diametrically large portion alone of individual rivet is undesirable for the good productivity, and should the diametrically small portions of a rivet be inadvertently heated, then a rupture would develop at the rivet head at the time of formation of the rivet head.